Recently, the growing interest in the environment and energy all over the world is encouraging research into improving fuel efficiency in the automobile industry. Research and development into reducing the weight and size and increasing performance have been being steadily conducted to fulfill different kinds of consumer demands. Particularly, research and development into hybrid vehicles which use engine power and electric energy together are increasing.
Many kinds of hybrid vehicles use an idle stop/go system in which an engine automatically stops when a vehicle stops, for example, while waiting for the light to change, and manipulating a transmission restarts the engine. However, because such a hybrid vehicle also uses the engine to operate the air-conditioning apparatus, when the engine is stopping, a compressor also stops, increasing the temperature of an evaporator, resulting in making a user feel unpleasant. Furthermore, refrigerant in the evaporator easily evaporates even at room temperature. Hence, if the refrigerant evaporates during the time for which the compressor is not in operation, even though the engine is restarted, and the compressor and the evaporate are re-operated, it takes a relatively long time to supply cold air into the passenger compartment because it is necessary to compress the evaporated refrigerant and re-liquefy it. In addition, this causes the problem of an increase of the overall energy consumption.
In an effort to improve air-conditioning efficiency, a related technique was proposed in Japanese Patent Laid-open Publication No. 2000-205777 (entitled: Cold-storage heat exchanger). This technique is illustrated in FIG. 1.
As shown in FIG. 1, the cold-storage heat exchanger 90 includes tubes 91, each of which has a double tube structure and is integrally configured such that a refrigerant passage 91e through which refrigerant flows and cold-storage-medium chambers 91f and 91f are defined in the double-layered tube 91. Fluid passages 94 are formed between the tubes 91 an that heat can be transferred between the refrigerant and fluid passing through the fluid passages 94.
However, in the conventional cold-storage heat exchanger of FIG. 1, manufacturing the double-layered tube includes joining several sheets of plates. Defective joining frequently occurs. The double tube structure makes the manufacture of the tube difficult. The problem of defective joining is accompanied by a problem of the refrigerant mixing with the cold-storage medium. In addition, even if there is a defectively joined portion, it is very difficult to find such a portion.
Moreover, although the conventional cold-storage heat exchanger has an advantage in that the cold-storage medium receives cold energy from the refrigerant because the passage along which the refrigerant flows is disposed at an inside position in the double tube structure while the cold-storage-medium chamber that stores the cold-storage medium is disposed at an outside position in the double tube structure, this double tube structure reduces the efficiency of heat transfer between the refrigerant and air that passes over the outside of the double tube structure and comes into contact with the cold-storage-medium chamber. Further, the fins that are disposed outside the double-layered tubes also come into contact only with the cold-storage-medium chamber without making direct connect with the refrigerant passage. In addition, this double tube structure restricts the size of the space in which the cold-storage medium can be stored, thus resulting in reducing the heat exchange efficiency.
FIG. 2 illustrates another conventional technique, showing a plate type cold-storage heat exchanger 80 which has a structure of three rows with a cold-storage medium stored in a middle row. In this cold-storage heat exchanger, a part for storing a cold-storage medium is disposed between front and rear rows of tubes 81 each of which is formed by joining plates 82. This conventional technique has the advantage of improving the efficiency with which cold energy of the refrigerant is stored in the cold-storage medium, but given the characteristics of the plate type, it is difficult to provide a sufficient space for storing the cold-storage medium. Therefore, the amount of cold-storage medium is insufficient, making it difficult to ensure satisfactory cold-energy storage performance. Furthermore, the durability and the corrosion resistance are comparatively low, reducing the lifetime of the cold-storage heat exchanger.